1. Field of the Invention
The present invention relates to a wireless communication system. More particularly, the present invention relates to techniques for supporting a Low Duty Cycle (LDC) mode of a Base Station (BS) of a wireless communication system.
2. Description of the Related Art
In a typical wireless communication system, a service, such as a voice and/or data service, is provided to Mobile Stations (MSs) through a plurality of Macrocell Base Stations (MBSs). Each of the MBSs is responsible for providing a service to MSs located within their respective service coverage area. The service coverage area of an MBS is hereafter referred to as a Macrocell. To facilitate mobility of the MSs, handoff between MBSs is performed when the MS leaves one Macrocell for another.
In the wireless communication system, a channel may deteriorate due to a number of factors, including a geographical factor inside a Macrocell, a distance between an MS and an MBS, movement of the MS, etc. The channel deterioration is problematic since it may result in a disruption of communication between the MS and the MBS. For example, when the MS is located inside a structure, such as an office building or a house, a channel between the MBS and the MS may deteriorate due a shadow region that is formed by the structure. A shadow region formed within the structure is hereafter referred to as an indoor shadow region. The MS located in the indoor shadow region may not be able to adequately perform communication with the MBS. Further, an MBS may have inadequate capacity to service all users in its Macrocell. To address the shortcomings of the MBS, a variety of other Base Stations (BSs) have been proposed, including a Relay, Picocell, Microcell, Femtocell, Ubicell etc. Since the Femtocell is a more recent concept, the Femtocell concept will be explained further below.
The Femtocell concept is proposed to provide ubiquitous connectivity to MSs and improve wireless capacity, while addressing a service limitation of an indoor shadow region. A Femtocell is a small cell coverage area that is serviced by a low power Femtocell BS (FBS) that accesses a wireless communication Core Network (CN) via a commercial broadband network or via a wireless connection to a backhaul of the wireless communication system. Hereafter, the term “Femtocell” may be used interchangeably with the term “FBS.” The deployment of the Femtocell improves both the coverage and capacity of the wireless communication system. Going forward, the advantages of the Femtocell are expected to be increasingly leveraged in wireless communication systems. Since the Femtocell is much smaller than a Macrocell, a plurality of Femtocells may exist within one Macrocell. The FBS is capable of providing service to-relatively a small number of MSs, unlike the MBS, which is capable of providing service to a large number of MSs. The FBS typically operates in a licensed spectrum and may use the same or different frequency as the MBS. Further, MSs serviced by an FBS are typically stationary or moving at low (i.e., pedestrian) speed. Similar to the need for handoff between MBSs, handoff between the Femtocell and the Macrocell, and handoff between Femtocells, is a key function in securing service continuity of an MS.
The FBS may be installed inside or adjacent to an indoor space to which it is intended to provide service, such as a home or Small Office/Home Office (SOHO). Installation of the FBS is significantly easier than installation of the MBS and the FBS may be purchased and installed by a subscriber for use in conjunction with the wireless communication system. Here, the subscriber or service provider may desire to limit access to the FBS and only provide access to authorized MSs. To facilitate this arrangement, a Closed Subscriber Group (CSG) FBS may be employed. The CSG FBS can be further categorized as a CSG-closed and a CSG-open FBS. A CSG-closed FBS may only provide access to authorized MSs, except for emergency services and National Security/Emergency Preparedness (NS/EP) services. A CSG-open FBS may, in addition, allow non-subscribers of the CSG a low priority access or a best-effort access, to guarantee the Quality of Service (QoS) of the subscribers. Alternatively, an Open Subscriber Group (OSG) FBS may be employed that provides access to any MS.
The FBS has different operational requirements than the MBS, the specifics of which may differ depending on the wireless communication system it is implemented in. For example, in a wireless communication system operating according to the Institute of Electrical and Electronics Engineers (IEEE) 802.16m standard, it is required that the air interface support features needed to limit MSs scanning for FBSs. Further it is required that the air interface support MSs in being able to access and handover to CSG FBSs if they are designated as part of the CSG. In addition, it is required that the air interface supports preferred access and handover of mobile stations to their designated FBSs. Also, it is required that the air interface supports optimized and seamless session continuity and handover between FBSs and Wireless Fidelity (WiFi) access systems. Further, it is required that the air interface allows for a dense deployment of a large number of Femtocells in a wireless communication system.
Given the above requirements for the Femtocell, which differ from the requirements of the Macrocell, there are a number of issues that arise with the deployment of the Femtocell in a wireless communication system. For example, when Femtocells are densely deployed, interference may be introduced from the Femtocells to Macrocells and to other Femtocells. This is particularly evident in the case of the CSG FBS, which only provides service to authorized MSs. When an unauthorized MS enters the Femtocell of the CSG FBS, the unauthorized MS will receive a strong signal from the CSG FBS. However, since the MS is unauthorized, the MS will not be able take advantage of the strong signal of the CSG FBS for service. Thus, the signal ends up being strong interference to the unauthorized MS. To reduce the interference to others, the FBS may enter a Low Duty Cycle (LDC) mode in different situations, for example, when there are no MSs in its Femtocell, when there is no active MS or connected MS in the Femtocell, when all the MSs attached to the Femtocell are in idle mode or sleep mode, etc. When in the LDC mode, the FBS may be available—or active in limited resources such as time and/or frequency in the air interface.
The concept of an LDC mode is a new. Such a concept for Macrocells has not previously been investigated because it is very rare that a Macrocell does not have any active or prospective MSs within its Macrocell. However, the situation is different for FBSs, where it is common not to have any active or prospective MSs in its Femtocell. For example, consider a situation where a user leaves his home, in which an FBS is disposed. In this case, the LDC mode may be employed to reduce power consumption of the FBS and may reduce the interference caused by the FBS to other entities. While the LDC mode is particularly advantageous for Femtocells, the LDC mode may also be advantageous for other BSs, including Relays, Picocells, Microcells, Macrocells, etc.
While the benefits of implementing the LDC mode are evident, there are a number of issues regarding the LDC mode that need to be addressed in order to support the LDC mode of a BS, such as an FBS. Exemplary issues that need to be addressed include the leaving and entering of the LDC mode by the BS, the waking up of the BS in the LDC mode, and how the MS will scan, monitor, and perform Network Entry (NE) to the BS in the LDC mode. These issues, as well as others, are new and need to be resolved. Some of these issues have been discussed with respect to Femtocells, as will be described below.
With respect to Femtocells, it has been suggested that, to perform NE to Femtocells in the LDC mode, the MS should know the resource (e.g., time, frequency) assigned to the FBS, such as the timing of an Available Interval (AI) of the LDC mode of the corresponding FBS, and carrier frequency. However, no specific technique has been proposed to achieve this. Further, with respect to Femtocells, it has been suggested that different CSG FBSs within the same Macrocell should become available at non-overlapping time slots, in order to hide themselves from unauthorized MSs. However, such a scheme may not be effective since an MS in active communication will scan for all FBSs, including the CSG FBSs using the non-overlapping active time slots, despite the MS not being authorized to receive service from those FBSs. In addition, while the scheme may be useful to reduce interference among CSG Femtocells, there may not be a need for all Femtocells within the same Macrocell to be in different active slots, because only neighboring Femtocells cause the femto-to-femto interference.
In addition, with respect to Femtocells, it has been suggested that the MS may wake up an FBS in the LDC mode by sending a wake-up ranging code, wherein the wake-up ranging code is a ranging code that has been reserved for this purpose. However, how to reserve such ranging code systematically and efficiently has not yet been proposed. Further, how exactly to perform NE to Femtocells in the LDC mode has not been proposed.
Moreover, with respect to Femtocells, it has been suggested that the MBS should broadcast Frequency Allocation (FA) and scrambling codes of FBSs within their Macrocell in order to assist the MS in finding a Femtocell. While it is beneficial for the MS, which is searching for a Femtocell, to know the FAs of Femtocells, the benefit is mitigated for Femtocells in the LDC mode. To find Femtocells in the LDC mode, it would be beneficial for the MS to know the starting time and length of the AI of the LDC mode of the Femtocells. Knowing the starting time and length of the AI of the LDC mode of the Femtocells would enable the MS to expedite searching for Femtocells in the LDC mode as well as increase the chance of finding the Femtocells in the LDC mode. However, it has not been suggested how to utilize the information of available timing and interval of Femtocells to assist MSs in their search for Femtocells.
Accordingly, despite the issues regarding the LDC mode being discussed with respect to Femtocells, the issues regarding the LDC mode of how to assist an MS to efficiently scan and search for a BS in the LDC mode, how to perform NE and handover to a BS in the LDC mode, how and when to wake up a BS in the LDC mode, and so on, remain to be addressed.
Therefore, a need exists for techniques to support the LDC mode of a BS of a wireless communication system.